1. Field of the Invention
The present invention relates to a touch-sensing circuit, and in particular to a touch-sensing circuit arranged to detect a touch event according to the reference voltages corresponding to different periods.
2. Description of the Related Art
Presently, mobile devices are highly developed and multi-functional. For example, handheld devices such as mobile phones or tablets are capable of conducting telecommunications, receiving and transmitting e-mails, maintaining social networks, managing contacts, and playing media. Hence, users can implement various applications on their mobile devices, such as a simple phone call, social network interaction, or commercial transaction. Therefore, mobile devices have become one of the necessities of people's lives.
Most of the current handheld devices include touch screens and touch units. Users may input information and commands using it. FIG. 1 is a common touch-sensing circuit 100. The common touch-sensing circuit 100 includes a resistor R0, a capacitor C0, a comparator COM0 and a switch SW0. The resistor R0 has a first terminal coupled to a voltage source VDD, and a second terminal connected to a first input terminal of the comparator COM0. The capacitor C0 has a first terminal connected to the first input terminal of the comparator COM0, and a second terminal connected to a ground GND. The comparator COM0 is arranged to determine whether the capacitor C0 is fully charged by the voltage source VDD according to a reference voltage Vref provided by a second input terminal and a signal received by the first input terminal. When the capacitor C0 is fully charged, the comparator COM0 is arranged to enable the switch SW0 to be connected to the ground GND to discharge the capacitor C0.
When no touch event occurs, the capacitor C0 needs a first period to be fully charged. When a touch event occurs, the capacitance of the capacitor C0 is increased, and the capacitor C0 needs a second period which is longer than the first period to be fully charged. Therefore, the touch-sensing circuit 100 can determine whether a touch event occurs according to the time that the capacitor C0 needs to be fully charged. However, the sensing circuit 100 needs a resistor R0 with large resistance to avoid noise. Furthermore, the sensing circuit 100 needs to detect whether the capacitor C0 is fully charged with a high frequency. Therefore, the sensing circuit 100 requires a lot of power.